The invention is directed to a medical device implantable into the body of a living being and comprising means for electrical stimulation of tissue contractions with adjustable stimulation intensity, a detector means for detecting stimulated tissue contractions, and setting means for automatically setting the stimulation intensity such that the detector means detects a stimulated tissue contraction after a stimulation. The invention is also directed to a method for the operation of such a device.
The term "stimulation intensity" is used comprehensively herein and includes, singly or in combination, the duration, frequency, repetition rate, amplitude, etc., with which the means for stimulating are activated.
The term "sensitivity" as used herein is to be understood as a threshold that a signal parameter or signal parameters of a signal corresponding to the physiological function that is detected, for example, the amplitude and/or steepness of an electrical signal, must exceed in order to lead to a detection. A high sensitivity therefore corresponds to a low threshold that must be exceeded, whereas a low sensitivity corresponds to a high threshold that must be exceeded.
In devices of the type to which the invention relates, the automatic setting of the stimulation intensity is carried out with the object of, first, assuring that every stimulation in fact leads to a stimulated tissue contraction and, second, guaranteeing that this is not achieved on the basis of an unnecessarily high stimulation intensity. A high stimulation intensity usually means high energy consumption. Since the normally provided energy source in an implantable medical device is a battery, and a battery has a limited stored power capacity, an unnecessary high stimulation intensity should be avoided under all circumstances so as to prolong the useful life of the battery and, thus, of the device.
A problem arises in such devices in that the detection of a stimulated tissue contraction can usually ensue only directly following a stimulation. At this time, however, the tissue charged with the stimulation energy is still polarized as a result of the application of electrical energy. There is thus the risk that the detector means will be overdriven, so that a proper detection of a stimulated contraction is not possible.
There is the possibility of only activating the detector means when a certain balancing of potential has already occurred; however, there is then the risk that a detection will no longer be possible at that time. Further, there is the possibility of undertaking a further energy application immediately following the stimulation, this producing a balancing of potential. Although a slight quantity of energy is required for the balancing of potential in comparison to the stimulation, this additional energy consumption is nonetheless undesirable because it has a negative influence on the useful life of the battery.